Yep, it's an interesting train of thought. I think first valid step would be for companies to start valuing open source contributions a lot more while hiring or giving freelance gigs. That creates a big incentive to contribute.

Open source projects can also maintain a list of easily fixable bugs that newbies can jump in and start contributing right away. They can also tell people to start documenting code (which is a valid contribution). An online code editor that makes me fix things right then and there would also be cool (instead of checking out a 2GB code and then fixing).

I feel most of the times API structures are decided in meetings on whiteboards and it becomes a pain maintaining context across such meetings.
Also with remote teams the same process is usually done over email.

Personally I haven't come across any tool which enables the entire team to collaborate on designing the API.

Oh, Go. I never did get the hang of it. Now, this is just me, and I know a lot of people like it, but somehow, never felt comfortable with Go. It did work out well for a few products, when they transitioned from Python-based stacks to Go, but it's not exactly gone viral. Unless you count Docker, which is completely written in Go. Now that was really good stuff.